Install PDF dependencies on demand in GitLab CI

Building the PDF version of the BIND 9 ARM requires TeX Live to be present on the build host. A TeX Live installation takes up several gigabytes of disk space. This significantly increases the size of the Debian Docker images that include that toolchain, even though only two GitLab CI jobs actually use it.

Instead of including TeX Live in the Docker image itself, install the former on demand in a new GitLab CI job that only tests building the PDF version of the BIND 9 ARM. This enables the size of the "base" Docker image (which a lot of GitLab CI jobs need to pull) to remain within reasonable limits. As downloading and installing TeX Live takes a significant amount of time, only run the new job in scheduled pipelines and for tags. Adjust job dependencies so that the "release" job continues to work.

See also !8350 (merged). This is the counterpart of that MR that I would propose to merge into v9.18 & v9.16, where we won't be touching the build system (but we still want the "base" image to be small).